Method of decarburizing electrical steel by using an oxide film



United States Patent 3,201,294 METHOD OF DECARBURIZING ELECTRICAL STEEL BY USING AN OXIDE FILM John Hawkins, Merewether, New South Wales, and Brian Lawrence Rowe, Whitehridge, New South Wales, Australia, assignors to John Lysaght (Australia) Limited, Sydney, New South Wales, Australia No Drawing. Filed Jan. 16, 1963, Ser. No. 251,719 Claims priority, application Australia, Jan. 19, 1962,

13,482/ 62 14 Claims. (Cl. 148-113) This invention relates to the heat treatment of electrical steels, and in particular to treatment whereby the steel is decarburized, annealed, stress relieved and provided with a surface film having electrical resistance characteristics.

As is well known, it is desirable that the carbon content of electrical steels be kept to a minimum in order that optimum magnetic properties (notably minimum core loss and maximum permeability) may be achieved. To this end during the processing of electrical steels either as sheet, coil or laminations, it is usual to effect an annealing treatment under such conditions that the steel is decarburized.

Although gaseous decarburizing media have been most often employed, the use of an oxide layer as a decarburizing medium is known. Decarburization takes place by virtue of the reaction between the oxides in this layer and the carbides in the steel and this reaction is typified by the following equation:

be taken of the commonly used practice of lubricating the feed to improve tool life; and scale chips which could be included in the electrical apparatus and which could damage bearings, insulation, etc., are not formed during punching.

In the case of feed produced from cold reduced electrical steels, hot band scale films are removed prior to cold reduction. The scale free surface of such cold reduced feed is ideal for punching and if the material is annealed,

after cold reduction, efforts are usually made to retain this surface by use of suitable protective atmospheres which may also be decarburizing.

Sheets produced from hot rolled electrical steeels are usually annealed while still carrying the scale films formed during hot rolling. While these films effectively decarburize the sheets, they are modified during the annealing treatment, and are difiicult to remove by pickling to achieve a scale free surface for punching.

The primary hot rolled scale is readily removed by pickling but this is normally at the expense of decarburization during annealing.

For optimum magnetic properties, it is usual to effect a heat treatment of the electrical steel laminations after punching to remove punching stresses. While the laminations could be decarburized during this treatment, hitherto it has not proved economical to perform such as operation during this treatment.

While the aforesaid scale free surface is the most suitable for the punching operation, its surface electrical resistance characteristics render it not suitable for use on laminations used in electrical apparatus. It is customary to provide a surface film on the laminations by heat-treatment or other means well known to those experienced in the art, having a suitable electrical resistance.

The primary object of the invention is to simplify the production of finished electrical steel laminations by the provision of a single heat-treatment to replace the more or less separate treatments used hitherto to decarburize the laminations and to develop theirmagnetic properties, either as such or as strip or sheet or the like prior to punching; to relieve the residual stresses introduced in the punchingof the lamination and finally toproduce an insulating oxide coating on the finished laminations.

Another main object of the invention is to provide a single heat-treatment as aforesaid which is applicable to clean laminations (that is laminations free from surface scale) and thus which enables clean feed to be used when laminations are being punched out.

Furthermore, it has been found that laminations produced by the method of the invention exhibit better magnetic properties than those of laminations of the'same compostion given a similar type of heat treatment in separate steps as has been the practice hitherto.

While it is obvious that if the single treatment accord-' ing to the invention i used, it is not necessary that the feed for lamination production be annealed. at any stage prior to that treatment, the invention is not limited to cases in which no prior annealing has occurred.

The term lamination as used hereinafter includes not only laminations fully shaped as required for their ultimate use but also partly shaped stock or blanks, such as sheets or strips from which such fully shaped laminations may be cut.

In principle, according to theinvention, the surfaces Broadly stated, the invention consists in a method of treating a scale free lamination of electrical steel having a silicon content within the range from 0 to 4.5% by weight comprising the following steps:

(a) Heating the untreated lamination to an oxidizing temperature below an annealing temperature and holding the lamination at said oxidizing temperature in an oxidizing atmosphere to form an oxide decarburizing medium on the surface of the lamination,

(b) Then heating the lamination to the said annealing temperature and holding the lamination at the said annealing temperature in a substantially inert atmosphere at least until the said decarburizing medium decarburizes the lamination and stresses in the lamination set up during the punch forming are relieved,

(c) Then causing or allowing the lamination to cool, and

(d) Applying an insulatory coating to the lamination.

The invention also ex'tendsto laminations when made by the method of the invention.

In a preferred example of the method of the invention the step of applying the insulating coating is carried out during the cooling phase of the stress relieving step, by holding the lamination at a second oxidizing temperature Patented Aug. 17, 1965.

below the annealing temperature in an oxidizing atmosphere for a period sufficient to produce an insulating oxide surface coating on the lamination. This second oxidizing temperature may or may not be the same as the first mentioned oxidizing temperature.

The various method steps according to preferred examples of the invention are further described as follows:

Step (0) A charge of clean laminations is packed into a suitable furnace and as the furnace comes up to oxidizing temperature an oxidizing atmosphere is admitted to the charge. This oxidizes the surface of the laminations forming an oxide layer which has been found to have suitable characteristics to act as a decarburizing medium. It has been found that an atmosphere containing water vapor at a dewpoint of 100 C. is a suitable oxidizing atmosphere and may be admitted at any constant oxidizing temperature between 200 C. and 600 C. or at any variable temperature between these limits. The time required for sufiicient oxidation to occur depends on the oxidizing temperature, the total surface area of the laminations to be treated and the amount of oxidizing constituents in the oxidizing atmosphere.

At the lower end of the oxidizing temperature range the rate of oxidation is slow and long times'are required to form an oxide film of sufficient thickness to adequately decarburize the laminations. On the other hand if the oxidizing medium is admitted at temperatures of 500 C. or greater, some sticking may occur if the laminations are annealed as a flat stack unless separating media are used. Consequently it is preferred for the oxidizing medium to be admitted at an oxidizing temperature in the range of from 400 C. to 500 C. to keep oxidation time to a minimum but still avoid the need for a separating medium. Above 600 C. the effectiveness of the oxide film formed is reduced and at 700 C. oxidation does not form a film of the required characteristics.

For a given oxidizing temperautre and total surface area of laminations the time for oxidation decreases as the amount of oxidizing constituents in the oxidizing atmosphere increases. In the preferred practice, the oxidizing atmosphere which is admitted to the charge is a water vapor bearing atmosphere having a dewpoint of 100 C. However, it is not essential that this oxidizing atmosphere be used. It has been found that any gas or mixture of gases which are oxidizing to the steel at the oxidizing temperature at which it is admitted will form an oxide layer of the required characteristics. Such gases and gas mixtures include, but are not limited to oxygen, water vapor, carbon dioxide or mixtures of any two or moreof these three gases with inert carrying gases, or mixtures of these and other oxidizing gases with any other gas or gases such that the resulting mixture is oxidizing to the laminations in the temperature range of 200 C. to 600 C.

The ease of diffusion of the oxidizing medium through a flat stack of laminations to react with the surfaces and form an oxide layer thereon is proportional to the width of solid sections in the laminations.

For laminations normally encountered in commercial practice, the diffusion of the oxidizing medium for the purpose of oxidation through the solid sections is adequate. If the laminations have relatively large solid sections, exceeding twelve inches or thereabouts, and are to be heat-treated according to the invention, the surfaces of the said laminations may be specially roughened or may be separated by suitable media to extend the range of diffusion of the oxidizing medium.

Step ([1) burization. It is therefore an important part of this invention that following the step of oxidizing the surface of the laminations the atmosphere surrounding the laminations be substantially non-reducing with respect to the oxides formed in the prior oxidation step. Furthermore, the atmosphere should also be non-carburizing with respect to the laminations. In the preferred practice, the substan tially non-reducing and non-carburizing atmosphere is nitrogen, although other inert gases such as argon are suitable. The aforementioned non-reducing and noncarburizing atmosphere can also be referred to as an inert atmosphere in view of the use of such inert gases as nitrogen and argon. Hydrogen contents of up to 2% can be tolerated in the nitrogen Without causing serious reduction of the oxide layers.

The decarburizing is effected at an annealing temperature which may be anywhere in the range of from 700 C. to 900 C. and may vary while decarburizing and annealing are in progress.

Generally the decarburizing will be completed before the lamination has been soaked at the annealing temperature for sufficient time to fully develop the inherent magnetic properties and to relieve strains set up in punching. Thus the annealing temperature is generally held for some further time after the decarburizing phase of this step.

While the lamination may be soaked at any temperature between 700 C. and 900 C. we prefer for general applications to use the range 750 C. to 850 C. when using nitrogen atmospheres in that the magnetic properties achieved are then an optimum over a wide range of inductions. Where special properties are required temperatures outside the preferred range but within the aforesaid limits of from 700 C. to 900 C. may be employed.

Steps (c) and (d) As mentioned earlier the final step (d) of coating the lamination with an insulating coating is preferably effected during the cooling step (c) and in this event the laminations, after soaking is finished, are cooled in what is still a substantially inert atmosphere until a second oxidizing temperature of, for example, 500 C. or thereabouts is reached when the surface of the lamination is again oxidized, in the preferred practice by water 'vapor. The purpose of this second oxidizing step is to form a surface film having a suflicient electrical resistance to minimize the components of iron losses due to eddy currents in normal commercial apparatus. This procedure of forming an oxide layer for this purpose is well known and widely used. However by following the procedure taught herein, the step of forming an oxide film for the reduction of eddy current loss is combined with the steps of decarburizing, relief of punching stresses and development of optimum magnetic properties.

By way of practical example of the invention, the presently preferred method of treatment of an eight ton charge of laminations is described hereinafter.

The laminations are stacked on racks, plates or containers or the like, all of which permit access of atmospheres to the laminations, and which are then stacked onto a suitable base. Alternatively the laminations may be packed directly onto the base. The charge may then be encased with a suitable gas tight container with provision for the introduction and discharge of atmospheres. The complete unit (that is the base, laminations and gas tight container) may then be charged into a furnace or if the base is fixed a suitable furnace may be placed in position over the base, laminations and gas tight container. If the furnace is so constructed that no gas tight container is required and the atmospheres may be admitted and discharged through the furnace proper, the gas tight container may be omitted.

Times at various temperatures will depend on charge volumes and as indicated above those quoted are for a batch annealed eight ton charge of laminations.

The charge is heated to 400 C. in the coldest portion whilst the hottest portion does not exceed 500 C. The atmosphere surrounding the charge during this period is nitrogen.

An atmosphere containing water vapor with a dew point of 100 C. is then admitted to the charge for five hours while the coldest portion of the charge is held between 400 C. and 460 C. and the hottest portion of the charge does not exceed 500 C.

On completion of the oxidizing step, nitrogen is passed through the charge at the rate of 2 volume changes/hour or at a suflicient rate to reduce the dew point of the atmosphere surrounding the laminations to C. or thereabouts by the time the laminations reach 600 C. Simultaneously with the introduction of the nitrogen the temperature of the charge is raised until the coldest portion of the charge reaches 750 C. The charge is then held for ten hours with the coldest portion of the charge in the temperature range 750 C. to 800 C. while the hottest portion does not exceed 850 C.

The charge is then caused to cool by either discharge from or removal of the furnace or by causing the furnace to cease heating. When the hottest portion of the charge reaches 500 C., the nitrogen atmosphere is discontinued and an atmosphere containing Water vapor at 100 C. dew point admitted to the charge for five hours. During this period the coldest temperature should not drop below 200 C. After this oxidation step is completed, the charge 'is uncovered and allowed to cool in 'air.

While the preferred practice is for batch annealing, by suitably adjusting times at temperature, a continuous furnace can be used.

While we have described our preferred practice and the scope of our invention, those experienced in the art may make modification to or departure from our practice without departing from the spirit of our invention.

In this regard, the oxidizing treatment for the purposes of forming an oxide film to decarburize the laminations may be carried out separately to the full annealing treatment in which decarburization occurs and the optimum magnetic properties developed. Also the insulating coating may be a varnish or other conventional insulating medium adapted to form a thin film on the lamination when painted or spread thereon and allowed to dry or set.

What is claimed is:

1. A method of making an electrical steel lamination from a scale-free portion of lamination stock of electrical steel having a silicon content up to 4.5% by weight, comprising the steps of punch forming a lamination from the lamination stock, heating the lamination to an oxidizing temperature within the range of from 200 C. to 600 C., holding the lamination at said oxidizing temperature in an oxidizing atmosphere to form an oxide decarburizing medium on the surface of the lamination, heating the lamination in an inert atmosphere to an annealingtemperature within the range of from 700 C. to 900 C. and holding the lamination at said annealing temperature and in said inert atmosphere at least until the said decarburizing medium decarburizes the lamination and relieves the stresses set up during the punch forming and cooling the lamination.

2. A method of making an electrical steel lamination from a scale-free portion of lamination stock of electrical steel having a silicon content up to 4.5% by weight, comprising the steps of punch forming a lamination from the lamination stock, heating the lamination to an oxidizing temperature within the range of from 200 C. to 600 C., holding the lamination at said oxidizing temperature in an atmosphere containing water vapor of a dew point of 100 C., to form an oxide decarburizing medium on the surface of the lamination, heating the lamination in an inert atmosphere to an annealing temperature within the range of from 700 C. to 900 C. and holding the lamination at said annealing temperature and in said inert atmosphere at least until the said decarburizing medium decarburizes the lamination and relieves the stresses set up during the punch forming and cooling the lamination.

comprising the steps ofpunch forming a lamination from the lamination stock, heating the lamination to an oxidizing temperature within the range of from 400 C.

to 500 C., holding the lamination at said oxidizing temperature in an atmosphere containing Water vapor of dewpoint of C. to form an oxide decarburizing medium on the surface of the lamination, heating the lamination in an inert atmosphere to an annealing temperature Within the range of from 700 C. to 900 C. and holding the lamination at said annealing temperature and in said inert atmosphere at least until the said decarburizing medium decarburizes the lamination and re- 'lieves the stresses set up during the punch forming and cooling the lamination.

4. A method of making a electrical steel lamination from a scale-free portion of lamination stock of electrical steel having a silicon content up to 4.5% by weight, comprising the steps of punch forming a lamination from the lamination stock, heating the lamination to an oxidizing temperature Within the range of from 200 C. to 600 C., holding the lamination at said oxidizing temperature in an oxidizing atmosphere to form an oxide decarburizing medium on the surface of the lamination, heating the lamination in an inert atmosphere to an annealing temperature within the range of from 750 C. to

850 C. and holding the lamination at said annealing temperature and in said inert atmosphere at least until the said decarburizing medium decarburizes the lamination and relieves the stresses set up during the punch forming and coolingthe lamination.

i 5. A method of making an electrical steel lamination from a scale-free portion of lamination stock of electrical steel having a silicon content up to 4.5% by weight, comprising the steps of punch forming a lamination from the lamination stock, heating the lamination to an oxidizing temperature within the range of from 200 C. to 600 C., holding the lamination at said oxidizing temperature in an atmosphere containing water vapor of a dewpoint of 100 C., to form an oxide decarburizing medium on the surface of the lamination, heating the lamination in an inert atmosphere to an annealing temperature within the range of from 750 C. to 850 C. and holding the lamination at said annealing temperature and in said inert atmosphere at least until the said decarburizing medium decarburizes the lamination and relieves the stresses set up during the punch forming and cooling the lamination.

6. A method of making an electrical steel lamination from a scale-free portion of lamination stock of electrical steel having a silicon content up to 4.5% by Weight, comprising the steps of punch forming a lamination from the lamination stock, heating the lamination to an oxidizing temperature within the range of from 400 C. to 500 C., holding the lamination at :said oxidizing temperature in an atmosphere containing water vapor of dewpoint of 100 C. to form an oxide decarburizing medium on the surface of the lamination, heating the lamination in an inert atmosphere to an annealing temperature within the range of from 750 C. to 850 C. and holding the lamination at said annealing temperature and in said inert atmosphere at least until the said decarburizing medium decarburizes the lamination and relieves the stresses set up during the punch forming and cooling the lamination.

7. A method of making an electrical steel lamination from a scale-free portion of lamination stock of electrical steel having a silicon content up to 4.5% by weight, comprising the steps of punch forming a lamination from the lamination stock, heating the lamination to an oxidizing temperature Within the range of from 200 C. 600 C., holding the lamination at said oxidizing temperature in an oxidizing atmosphere to form an oxide decarburizing medium on the surface of the lamination, heating the lamination in an atmosphere of nitrogen to an annealing temperature Within the range of from 700 to 900 C. and holding the lamination at said annealing temperature and in said atmosphere of nitrogen at least until the said decarburizing medium decarburizes the lamination and relieves the stresses set up during the punch forming and cooling the lamination.

8. A method of making an electrical steel lamination from a scale-free portion of lamination stock of electrical steel having a silicon content up to 4.5% by weight, comprising the steps of punch forming a lamination from the lamination stock, heating the lamination to an oxidizing temperature within the range of from 400 C. to 500 C., holding the lamination at said oxidizing temperature in an atmosphere containing water vapor of dewpoint of 100 C. to form an oxide decarburizing medium on the surface of the lamination, heating the lamination in an atmosphere of nitrogen to an annealing temperature within the range of from 700 C. to 900 C. and holding the lamination at said annealing temperature and in said atmosphere of nitrogen at least until the said decarburizing medium decarburizes the lamination and relieves the stresses set up during the punch forming and cooling the lamination.

9. A method of making an electrical steel lamination from a scale-free portion of lamination stock of electrical steel having a silicon content up to 4.5% by weight, comprising the steps of punch forming a lamination from the lamination stock, heating the lamination to an oxidizing temperature within the range of from 400 C. to 500 C., holding the lamination at said oxidizing temperature in an atmosphere containing water vapor of dewpoint of 100 C. to form an oxide decarburizing medium on the surface of the lamination, heating the lamination in an atmosphere of nitrogen to an annealing temperature within the range of from 750 C. to 850 C. and holding the lamination at said annealing temperature and in said atmosphere of nitrogen at least until the said decarburizing medium decarburizes the lamination and relieves the stresses set up during the punch forming and cooling the lamination.

10. A method of making an electrical steel lamination from a scale-free portion of lamination stock of electrical steel having a silicon content up to 4.5% by weight, comprising the steps of punch forming a lamination from the lamination stock, heating the lamination to an oxidizing temperature within the range of from 200 C. to 600 C., holding the lamination at said oxidizing temperature in an oxidizing atmosphere to form an oxide decarburizing medium on the surface of the lamination, heating the lamination in an inert atmosphere to an annealing temperature within the range of from 700 C.

to 900 C. and holding the lamination at said annealing temperature and in said inert atmosphere at least until the said decarburizing medium decarburizes the lamination and relieves the stresses set up during the punch forming and cooling the lamination.

11. A method according to claim 10 wherein the second oxidizing temperature is within the range of from 200 C. to 500 C.

12. A method of making an electrical steel lamination from a scale-free portion of lamination stock of electrical steel having a silicon content up to 4.5% by weight, comprising the steps of punch forming a lamination from the lamination stock, heating the lamination to an oxidizing temperature within the range of from 400 C. to 500 C., holding the lamination at said oxidizing temperature in an atmosphere containing water vapor of dewpoint of C. to form an oxide decarburizing medium on the surface of the lamination, heating the lamination in an atmosphere of nitrogen to an annealing temperature within the range of from 750 C. to 850 C. and holding the lamination at said annealing temperature and in said atmosphere of nitrogen at least until the said decarburizing medium decarburizes the lamination and relieves the stresses set up during the punch forming and cooling the lamination.

13. A method according to claim 12 wherein the said second oxidizing temperature is Within the range of from 200 C. to 500 C.

14. A method of making an electrical steel lamination from a scale-free portion of lamination stock of electrical steel having a silicon content up to 4.5% by Weight, comprising the steps of punch forming a lamination from the lamination stock, heating the lamination to an oxidizing temperature within the range of from 400 C. to 500 C., holding the lamination at said oxidizing temperature in an atmosphere containing water vapor of dewpoint of 100 C. to form an oxide decarburizing medium on the surface of the lamination, heating the lamination in an atmosphere of nitrogen to an annealing temperature within the range of from 750 C. to 850 C. and holding the lamination at said annealing temperature and in said atmosphere of nitrogen at least until the said. decarburizing medium decarburizes the lamination and relieves the stresses set up during the punch forming and cooling the lamination.

References Cited by the Examiner UNITED STATES PATENTS 1,842,162 1/32 Gifford 148113 2,158,065 5/39 Cole et al. 1481l3 2,227,156 12/40 Rear-don 148-113 2,264,859 12/41 Ruder 148-113 2,303,343 12/42 Engel et al. 148111 2,875,113 2/59 Fitz 148113 DAVID L. RECK, Primary Examiner. 

1. A METHOD OF MAKING AN ELECTRICAL STEEL LAMINATION FROM A SCALE-FREE PROTION OF LAMINATION STOCK ELECTRICAL STEEL HAVING A SILICON CONTENT CONTENT UP TO 4.5% BY WEIGHT, COMPRISING THE STEPS OF PUNCH FORMING A LAMINATION FROM THE LAMINATION STOCK, HEATING THE LAMINATION TO AN OXIDIZING TEMPERATURE WITHIN THE RANGE OF FROM 200*C. TO 600*C., HOLDING THE LAMINATION AT SAID OXIDIZING TEMPERATURE IN AN OXIDIZING ATMOSPHERE TO FORM AN OXIDE DECARBURIZING MEDIUM ON THE SURFACE OF THE LAMINATION, HEATING THE LAMINATION IN AN INERT ATMOSPHERE TO AN ANNEALING TEMPERATURE WITHIN THE RANGE OF FROM 700*C. TO 900*C. AND HOLDING THE LAMINATION AT SAID ANNEALING TEMPERATURE AND IN SAID INERT ATMOSPHERE AT LEAST UNTIL THE SAID DECARBURIZING MEDIUM DECARBUTIZES THE LAMINATION AND RELIEVES THE STRESSES SET UP DURING THE PUNCH FORMING AND COOLING THE LAMINATION. 